Direct methanol fuel cell operable with neat methanol

ABSTRACT

A fuel cell system running on direct neat methanol. Back diffusion of water from the cathode to the anode is sufficiently high so that water is not accumulated at the cathode, thereby leading to fuel cell systems without the need for a pump system to remove circulate water from the cathode to the anode. Other embodiments are described and claimed.

PRIORITY CLAIM

This application is a continuation of U.S. patent application Ser. No.12/008,106 filed Jan. 8, 2008, which claimed the benefit of U.S.Provisional Application No. 60/879,257, filed 8 Jan. 2007.

GOVERNMENT INTEREST

The claimed invention was made in the performance of work under a NASAcontract, and is subject to the provisions of Public Law 96-517 (35 USC202) in which the Contractor has elected to retain.

FIELD

The present invention relates to fuel cells.

BACKGROUND

FIG. 1 illustrates a high-level system diagram of a prior art directmethanol fuel cell operating on an aqueous feed of methanol. Neat (100%)methanol is stored in container 102 and then diluted with water to aconcentration of 2-3% before it is introduced into fuel cell stack 104.The methanol fuel solution is re-circulated, indicated by the loopcomprising flow line 106, gas and liquid separator 108, radiator 110 andbypass 112, pump 114, mixer 116, methanol sensor 118, and cold-startheater 120. Methanol is added to the solution by way of pump 114 andmixer 116 as needed to maintain the required concentration delivered toanode 122. The fuel solution entering anode 122 is accurately monitoredand controlled using methanol sensor 118.

The water used for this dilution is gathered at cathode 124, flowsthrough flow line 126 to sump tank 128, and is pumped by sump pump 130to gas and liquid separator 108. Carbon dioxide is generated at andremoved from anode 122, as indicated by flow line 132.

Diluting methanol, collecting and circulating water, circulating fuel,and controlling concentration entail the use of several pumps andcontrol systems, with their resulting use of electrical energy, and addto the size and cost of the fuel cell system. These auxiliary componentsmay constitute about 50% of the overall volume and mass of presentstate-of-art direct methanol fuel cell systems, and may contribute to atleast 50% of the parasitic energy consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art fuel cell system.

FIG. 2 illustrates a fuel cell according to an embodiment.

FIG. 3 illustrates a fuel cell system according to an embodiment.

FIG. 4 illustrates dependence of current density upon membrane thicknessfor an embodiment.

FIG. 5 illustrates an anode electrode according to an embodiment.

DESCRIPTION OF EMBODIMENTS

In the description that follows, the scope of the term “someembodiments” is not to be so limited as to mean more than oneembodiment, but rather, the scope may include one embodiment, more thanone embodiment, or perhaps all embodiments.

These letters patent teach the design of a direct methanol fuel cell inwhich back diffusion of water from the cathode to the anode in amethanol fuel stack is high enough so that water need not be collectedat the cathode, and neat methanol may be provided to the anode withoutthe need for dilution. By not requiring the gathering of water at thecathode, or the dilution of neat methanol, it is expected thatembodiments may have reduced parasitic energy losses, higher powerdensity, and higher reliability at reduced cost, compared to the priorart direct methanol fuel cell system of FIG. 1.

FIG. 2 illustrates an embodiment fuel cell 200, and some of theprocesses involved. During fuel cell operation neat methanol (CH₃OH) isoxidized at anode 202 to protons and carbon dioxide, and oxygen (0 ₂) isreduced to water at cathode 204. As protons migrate from anode 202 tocathode 204 by way of membrane 206, water is transported with theprotons (H⁺), as indicated by arrow 208. This process is termedelectro-osmotic drag, in which water molecules associated with theprotons are dragged across membrane 206 in the direction of ionicmovement.

Water is consumed at anode 202 by the oxidation of methanol, and wateris produced at cathode 204 by reduction of oxygen. Air flowing acrosscathode 204 evaporates some of this water. The remaining water generatedat cathode 204, as well as the water arriving at cathode 204 due toelectro-osmotic drag, is brought to anode 202 by to back diffusion, asindicated by arrows 208 and 210.

It is useful to understand the factors that govern the rates of variouswater transport processes. The rate of consumption of water at anode 202is determined by the current density, and for some embodiments, one moleof water is consumed for every 6 Faradays of electricity. The rate ofmigration of water from anode 202 to cathode 204 by electro-osmotic dragis determined by the current density and the drag coefficient. The someembodiments, the drag coefficient is about 3 molecules of water perproton. The rate of production of water at cathode 204 by reduction isalso determined by the current density, and for some embodiments isgiven by 0.5 mole of water for every Faraday of electricity.

The flow rate of air over cathode 204, the temperature of fuel cell 200,and the absolute humidity of air at the specified temperature determinethe rate of evaporative loss. The concentration gradient of waterbetween anode 202 and cathode 204, and the diffusion coefficient ofwater in the membrane-electrode composite (206) determine the rate ofback-diffusion. Accordingly, the rate of back diffusion may be increasedby increasing the concentration gradient for water, and this process maybe relatively independent of the current density.

Thus, the rate of back diffusion to move water from cathode 204 to anode202 that has not been removed due to evaporation may be achieved byappropriately choosing the concentration of methanol, the porosity ofthe electrodes in anode 202 and cathode 204, the thickness of membrane206, the operating temperature of fuel cell 200, and the stoichiometricrate of airflow over cathode 204. For any given set of conditions forthe foregoing variables, there will be a current density at which waterbalance will be achieved. Water balance means that the back diffusionrate and evaporative rate at cathode 204 are such that water does notaccumulate at cathode 204.

Calculations have shown that when about 1 molar (3%) methanol solutionsare circulated past anode 202, the rate of back diffusion is inadequateto transport the water for a practical value of current density, and thecurrent density at which water balance is achieved is quite low to be ofpractical value for most applications. However, when neat (100%)methanol is used, the back diffusion process alone is expected toachieve the water balance at a current density of 170 mA/cm²(mill-Ampere per square cm). This latter value of current density isexpected to be in the range useful for practical applications.

More specifically, calculations show that for a membrane thickness of0.0635 cm, a stoic rate for air flow over cathode 204 of 3.0, a dragcoefficient of 3.0 water molecules for each proton (H⁺) transported byelectro-osmotic drag, a temperature of 45° C., and a diffusioncoefficient for water of 8.16*10⁻⁶ cm²/sec; water balance is achievedfor a methanol concentration of 3% at a current density of only 7mA/cm², but water balance is achieved using neat methanol according toan embodiment at a useful current density of 170 mA/cm².

Thus, the above description teaches that by using neat methanol anddesigning the fuel cell such that water is returned to the anode by backdiffusion from the cathode, a practical current density may be achieved,and mechanical means for collecting and returning water may be avoided.FIG. 3 illustrates an embodiment, where neat methanol stored incontainer 302 is pumped by pump 304 to delivery system 306. Deliverysystem 306 provides neat methanol directly to anode 202, and fuel cell300 is designed so that the back diffusion of water is sufficient sothat water need not be removed at cathode 204, other than byevaporation. An embodiment for delivery system 306 is described later.

For some embodiments, membrane 206 may comprise Nafion. Nafion is asulfonated tetrafluorethylene copolymer, and is a registered trademarkof E. I. Du Pont de Nemours and Company, a corporation of Delaware. Forsuch embodiments, membrane 206 is hydrophilic, and may for someembodiments allow water retention of up to about 40% of the membranemass. Also, for some embodiments, water and carbon dioxide is producedby direct reaction of methanol with oxygen at anode 306 or cathode 204,so that an adequate supply of water may be produced.

If anode 202, cathode 204, or both are not capable of sustaining desiredcurrent densities due to slow catalysis or mass transport of reactants,then the electrode structures for anode 202 or cathode 204, and thethickness of membrane 206, may be adjusted so that an acceptable currentdensity value may be achieved. An example is illustrated by FIG. 4.

FIG. 4 illustrates a functional relationship between fuel cell currentdensity (mA/cm²) and membrane thickness (cm) for the following fuel cellparameters: a stoic air flow rate of 3.0; a drag coefficient of 3.0water molecules per proton; and a diffusion coefficient for water of8.16×10⁻⁶ cm²/sec. As noted in FIG. 4, reducing the membrane thicknessmay lead to an increase in current density. For example, whereas amembrane thickness of 0.2 cm provides for a current density of about 46mA/cm², reducing the membrane thickness to 0.06 cm provides for acurrent density of about 170 mA/cm².

The presence of an uncontrolled excess of neat methanol at anode 202 mayresult in swelling of membrane 206, which may lead to permanent damageof the membrane-electrode assembly. Accordingly, for some embodiments,the delivery rate of methanol to anode 202 should be such that only arelatively small quantity of methanol reaches the anode electrode.Furthermore, for some embodiments, the entire quantity of neat methanolthat is delivered to the anode electrode should be utilized within theelectrode structure, but the neat methanol should not reach the surfaceof membrane 206 in any significant quantity.

Full utilization of neat methanol at the anode electrode may be achievedif the electrode structure is modified to be porous and thick, so thatthe residence time for methanol is adequate for complete consumption inthe body of the electrode structure.

For some embodiments, such a porous electrode should have enough ionomermaterial to form conducting paths for the protons and water, but havethe enough tortousity to assure a high residence time.

An ionomer is a polyelectrolyte comprising copolymers. For someembodiments, the electrode structure may have layers of varying ionomercontent so that the desired level of utilization may be achieved. Thethickness, layer design, and porosity of the electrodes may depend onthe delivery rate. FIG. 5 illustrates in a simplified pictorial formanode electrode 502 and membrane 503. Anode electrode 502 comprisescarbonaceous substrate 504 and electrocatalyst layer 506. lonomermaterial is impregnated into carbonaceous substrate 504 to form varyinglayers of ionomer material 508, 510, 512, and 514.

For some embodiments, the optimization of the electrode structure shouldbe done in conjunction with the delivery method for the neat methanol.For some embodiments, delivery system 306 for delivering the neatmethanol to anode 202 may be an aerosol delivery system as described inU.S. Pat. No. 6,440,594. As another example, for some embodiments,delivery system 306 may include a diffusion barrier of sufficientthickness.

For some embodiments, the use of neat methanol at anode 202 and a backdiffusion that provides water balance may involve the use of a modifiedfuel cell stack design that incorporates methods not only for fueldelivery but also for heat removal. For some embodiments, circulatingfeed 308 may be used, so that excess heat may be removed from anode 202by way of radiator 310. Heat may also be removed by evaporative coolingon cathode 204. For some embodiments without circulating feed 308, heatloss due to evaporation at cathode 204 may be augmented with heatremoval by way of cooling fins 312 on the fuel stack. The design of suchfins may depend on the power level and other resources available forcooling.

Various modifications may be made to the described embodiments withoutdeparting from the scope of the invention as claimed below.

What is claimed is:
 1. A method of operating a fuel cell comprising:providing an anode and a cathode electrode in fluid communication via amembrane disposed therebetween; exposing the anode electrode to a sourceof methanol, such that the methanol is oxidized at the anode to produceprotons, carbon dioxide, and oxygen; exposing the cathode electrode to asource of air, such that the cathode reduces the oxygen from the anodeto produce water, and wherein the cathode electrode evaporates water ata characteristic water evaporation rate; wherein the membrane disposedbetween the anode electrode and the cathode electrode has a watermigration rate that characterizes the flow of water molecules from theanode electrode to the cathode electrode resulting from electro-osmoticdrag, and having a back diffusion rate that characterizes the flow ofwater molecules from the cathode electrode to the anode electrode;configuring the membrane fuel cell operating conditions such that duringoperation the water migration rate is substantially equal to the sum ofthe back diffusion rate and the water evaporation rate such that all thewater circulating within the fuel cell is produced at the cathodeelectrode from the reaction of the methanol at the anode electrode. 2.The method as set forth in claim 1, further comprising cooling the fuelcell.
 3. The method as set forth in claim 2, wherein the coolingcomprises the use of one of either a circulating radiator or evaporativecooling.
 4. The method as set forth in claim 1, further comprisinggenerating an aerosol of methanol for provision to the anode electrode.5. The method as set forth in claim 1, further comprising providing adiffusion barrier through which said methanol flows to reach the anodeelectrode.
 6. The method as set forth in claim 1, further comprisingimpregnating the anode electrode with ionomer material to providevarying layers of ionomer material.
 7. The method as set forth in claim1, wherein the membrane is hydrophilic.
 8. The method as set forth inclaim 1, wherein the membrane is capable of retaining water up to aconcentration of about 40% of the membrane mass during operation.
 9. Themethod as set forth in claim 1, further comprising configuring themembrane thickness to provide sufficient current density in said fuelcell.
 10. The method as set forth in claim 1, wherein the fuel cell isoperated at a current density of 170 mA/cm².
 11. The method as set forthin claim 1, wherein the membrane is formed from a sulfonatedtetrafluorethylene copolymer.
 12. A method of operating a fuel cellhaving an anode electrode and a cathode electrode in fluid communicationvia an interposing membrane comprising configuring the fuel cell suchthat a water migration rate that characterizes the flow of watermolecules from the anode electrode to the cathode electrode resultingfrom electro-osmotic drag is substantially equal to the sum of a backdiffusion rate that characterizes the flow of water molecules from thecathode electrode to the anode electrode and a water evaporation rate atthe cathode electrode.
 13. The method of claim 12, wherein all the watercirculating within the fuel cell is produced at the cathode from thereaction of methanol at the anode electrode.
 14. The method as set forthin claim 12, further comprising cooling the fuel cell.
 15. The method asset forth in claim 12, further comprising generating an aerosol ofmethanol for provision to the anode electrode.
 16. The method as setforth in claim 12, further comprising providing a diffusion barrierthrough which said methanol flows to reach the anode electrode.
 17. Themethod as set forth in claim 12, further comprising impregnating theanode electrode with ionomer material to provide varying layers ofionomer material.
 18. The method as set forth in claim 12, wherein themembrane is hydrophilic.
 19. The method as set forth in claim 12,wherein the membrane is capable of retaining water up to a concentrationof about 40% of the membrane mass during operation.
 20. The method asset forth in claim 12, further comprising configuring the membranethickness to provide sufficient current density in said fuel cell.